Lately, mastodon (and now lemmy!). It's also been super handy as a "swiss army knife" unix system. Need to test access outside my network? ssd -D to sdf. Need to copy files between two systems that would normally be a PITA? scp to and from sdf! Also nice to have access to a bsd machine since everything I personally run is Linux.
I've also used it in the past for Plan 9 learning and a couple other random things here and there.
I was blown away by that when I booted it on my 486 way back when. I was not quite skilled enough to get networking going under linux, and the modem I had was hell to setup in windows (plug and pray!)... Under QNX everything worked out of the box, and it ran circles around Win 95. Really impressive